In recent years, a slush molding method has widely been used for interior materials and the like of automobiles, since the molding makes it possible to mold easily into a product having a complicated form (such as an undercut or deep drawn form) and give an even wall thickness, and makes the yield percentage of the raw material good. Conventionally, in many cases, slush molding powdery products of a vinyl chloride-based resin have been used as skin materials of automobile interior components such as an instrument panel and a door trim.
However, when a vinyl chloride-based material is used over along term, a plasticizer contained therein shifts to the surface thereof in accordance with a use environment thereof, so that the material is damaged in soft feeling. When an automobile is disposed and then subjected to incineration treatment, hydrogen chloride gas is generated in accordance with the incineration temperature, so that the incineration furnace may be corroded in some cases.
In order to solve these problems, developments are being advanced about a slush molding powdery material comprising a thermoplastic resin other than vinyl chloride. Use is made of, for example, a thermoplastic polyurethane elastomer having a number-average molecular weight of 20,000 to 50,000 and a glass transition temperature of −35° C. or lower (Patent Document 1).
Use is also made of a granular polyurethane resin composition made mainly of a thermoplastic urethane resin synthesized in a non-aqueous dispersion medium, a polymer of a vinyl monomer, and a thermally crosslinkable monomer (Patent Document 2). Furthermore, use is made of a thermoplastic elastomer composition powder having a particle diameter of 1 to 1000 μm and comprising an acrylic block copolymer and an acrylic polymer having, in a single molecule thereof, reactive functional groups the number of which is 1.1 or more (Patent Document 3).
In a material for slush molding, a colored material is used as a skin material of an automobile interior component in order to cause the component to exhibit high quality feeling. These materials for slush molding, each comprising a thermoplastic resin, are also colored by synthesizing uncolored particles, and then dusting a coloring agent such as an inorganic pigment or an organic pigment onto the surfaces thereof. In this coloring method, however, by shear at the time of stirring and mixing in the step of the coloring or in a step subsequent thereto, particles of the pigment which adhere onto the resin particle surfaces are slipped down from the surfaces, or the pigment particles are aggregated with each other on the particle surfaces. Thus, this method has a problem that the original color of the coloring agent is not expressed, a lump of the pigment is intermingled with the resultant product, or the like.
In order to solve such a problem, suggested is a method in which before a resin is granulated, a liquid prepolymer for the resin is mixed with a coloring agent, and subsequently the mixture is granulated (Patent Document 4).